Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Mar 24, 2022 at 10:07 AM Toke Høiland-Jørgensen <toke@toke.dk> wrote:
>
> Right, but is that sync_for_device call really needed?

Well, imagine that you have a non-cache-coherent DMA (not bounce
buffers - just bad hardware)...

So the driver first does that dma_sync_single_for_cpu() for the CPU
see the current state (for the non-cache-coherent case it would just
invalidate caches).

The driver then examines the command buffer state, sees that it's
still in progress, and does that return -EINPROGRESS.

It's actually very natural in that situation to flush the caches from
the CPU side again. And so dma_sync_single_for_device() is a fairly
reasonable thing to do in that situation.

But it doesn't seem *required*, no. The CPU caches only have a copy of
the data in them, no writeback needed (and writeback wouldn't work
since DMA from the device may be in progress).

So I don't think the dma_sync_single_for_device() is *wrong* per se,
because the CPU didn't actually do any modifications.

But yes, I think it's unnecessary - because any later CPU accesses
would need that dma_sync_single_for_cpu() anyway, which should
invalidate any stale caches.

And it clearly doesn't work in a bounce-buffer situation, but honestly
I don't think a "CPU modified buffers concurrently with DMA" can
*ever* work in that situation, so I think it's wrong for a bounce
buffer model to ever do anything in the dma_sync_single_for_device()
situation.

Does removing that dma_sync_single_for_device() actually fix the
problem for the ath driver?

There's a fair number of those dma_sync_single_for_device() things all
over. Could we find mis-uses and warn about them some way? It seems to
be a very natural thing to do in this context, but bounce buffering
does make them very fragile.

                 Linus